In recent years, polymeric materials have been increasingly used in wider applications. Demand for better heat resistant properties of these polymeric materials have increased accordingly. In some uses, polymeric materials are exposed to high temperatures, causing them to soften and lose their strength, and decompose and deteriorate, producing such a problem that they cannot be used for long periods of time.
Epoxy resin compositions are used in various fields, including laminating, coating, adhesion, sealing, molding, and the like. But, for the reasons mentioned above, epoxy resin compositions containing bisphenol A-type epoxy resins, which are in wide use today, have become insufficient in providing the properties in demand.
In order to improve heat resistance, polyfunctional-type epoxy resins, such as novolak-type epoxy resins, trifunctional-type epoxy resins, and tetrafunctional-type epoxy resins have been used. But, these polyfunctional-type epoxy resins and their cured products do not have sufficient strength at high temperatures, and are rigid and brittle.
It is proposed in Polymer, Vol. 33, p. 2975 (1992) that when biphenol-type epoxy resins are cured with diaminodiphenylmethane, they have excellent mechanical properties at high temperatures; however, products obtained by curing with amine-type curing agents are known to have inferior thermal stability.
Products obtained by curing with phenolic curing agents are known to have excellent thermal stability. But, in the case of biphenol-type epoxy resins even when they are cured using common phenolic curing agents such as phenolnovolak there is room to improve the mechanical properties of the cured product at high temperatures.
Therefor it is an object of the present invention to provide an epoxy resin composition which when cured has high heat resistance, i.e., has excellent stability and strength at high temperatures.